Eric Eoin Marques, arrested in Ireland by the FBI in 2013, is the alleged operator of Freedom Hosting, a Web hosting service that authorities say enabled criminals to access child pornography and conduct other illegal activities on the Dark Web. The FBI called Marques the “largest child porn facilitator on the planet.” (AP Photo/Niall Carson)

Millions of people worldwide are using computer technology that allows them to visit websites, communicate with others and conduct business online without leaving a trace of their identity or location. That so-called anonymizing technology has created what experts call the Dark Web, a murky layer of the online world far less visible than the one accessible by Google and other common search engines. Proponents say the Dark Web's ability to mask identities helps protect dissidents in repressive regimes, allows police and military personnel to conduct covert operations and lets human rights activists report atrocities without risking reprisal. But critics say the Dark Web is a pathway for cybercrime, used by child pornographers, drug dealers and sex traffickers to hide their illegal dealings. Some law enforcement officials say the technology cripples their ability to catch criminals. But civil-liberties advocates counter that online anonymity is so valuable for good causes that it must not be curtailed.